This invention relates to a method of preparing monocrystalline gallium nitride thin films by electron cyclotron resonance microwave plasma assisted molecular beam epitaxy (ECR-assisted MBE). The invention further relates to a method for the preparation of n-type or p-type gallium nitride (GaN) films.
Efforts have been made to prepare monocrystalline GaN because of its potentially useful electrical and optical properties. GaN is a potential source of inexpensive and compact solid-state blue lasers. The band gap for GaN is approximately 3.4 eV, which means that it can emit light on the edge of the UV-visible region. For intrinsic GaN, the carrier concentration, n.sub.i, is 5.2.times.10.sup.3 cm.sup.-3, the mobility is 330 cm.sup.2 V.sup.-1 s.sup.-1 and the resistivity is 3.6.times.10.sup.12 .OMEGA.-cm.
Despite the desirability of a monocrystalline GaN film, its development has been hampered by the many problems encountered during the growth process. Previous attempts to prepare monocrystalline GaN films have resulted in n-type films with high carrier concentration. The n-type characteristic is attributed to nitrogen vacancies in the crystal structure which are incorporated into the lattice during growth of the film. Hence, the film is unintentionally doped with nitrogen vacancies during growth. Nitrogen vacancies affect the electrical and optical properties of the film.
ECR-assisted metalorganic vapor phase epitaxy gave GaN films that were highly conductive and unintentionally doped n-type (S. Zembutsu and T. Sasaki J. Cryst. Growth 77, 25-26 (1986)). Carrier concentrations and mobilities were in the range of 1.times.10.sup.19 cm.sup.-3 and 50-100 cm.sup.2 V.sup.-1 s.sup.-1, respectively. Efforts to dope the film p-type were not successful. The carrier concentration was reduced by compensation, that is, the effect of a donor impurity is "neutralized" by the addition of an acceptor impurity.
Highly resistive films were prepared by sputtering using an ultra-pure gallium target in a nitrogen atmosphere. The films were characterized n-type and the high resistivity was attributed to the polycrystalline nature of the films (E. Lakshmi, et al. Thin Solid Films 74, 77 (1977)).
In reactive ion molecular beam epitaxy, gallium was supplied from a standard effusion cell and nitrogen was supplied by way of an ionized beam. Monocrystalline films were characterized n-type, but higher resistivities of 10.sup.6 .OMEGA.-cm and relatively low carrier concentrations and mobilities (10.sup.14 cm.sup.-3 and 1-10 cm.sup.2 V.sup.-1 s.sup.-1, respectively) were obtained (R. C. Powell, et al. in "Diamond, Silicon Carbide and Related Wide Bandgap Semiconductors" Vol. 162, edited by J. T. Glass, R. Messier and N. Fujimori (Material Research Society, Pittsburgh, 1990) pp.525-530).
The only reported p-type GaN was a Mg-doped GaN treated after growth with low energy electron beam irradiation. P-type conduction was accomplished by compensation of n-type GaN (H. Amano et al. Jap. J Appl. Phys. 28(12), L2112-L2114 (1989)).
Current methods of preparing GaN do not permit control of nitrogen vacancies within the lattice. Thus it has not been possible to prepare intrinsic GaN. Additionally, it is desirable to control the doping process in GaN films, thereby enabling the production of p-n junctions. The present invention presents a method to prepare near-intrinsic monocrystalline GaN films and to selectively dope these films n- or p-type.